This invention relates to multi-stage catalytic systems for converting pollutants as NO.sub.x, CO, and hydrocarbons to innocuous products in which most or all of the rhodium is part of the first stage (partitioned upstream) and most or all of the platinum and/or palladium is part of the second stage (partitioned downstream). In one system, use of a zeolite in conjunction with rhodium as the first stage catalyst has advantages such as efficient conversion of the pollutants over a wider redox window with relatively low amounts of ammonia being generated. In another system, the upstream stage comprises a rapidly heatable substrate the purpose of which is to more efficiently catalyze the conversion of pollutants produced during cold-start or initial start-up of the pollutant generating process (such as automotive combustion) to innocuous products. Moreover, with the systems of the present invention, conversion is achieved with relatively low amounts of rhodium in the system.
Mixtures of oxides of nitrogen, commonly called NO.sub.x or more typically NO.sub.x gases, are generated as by-products in combustion processes such as in automotive engines or in fossil fuel power plants. These oxides are hazardous to health and the environment as they produce acid rain.
Up to the present time NO.sub.x emissions in automotive exhaust have been controlled by reducing them to nitrogen by a three way catalyst (TWC) such as Pt and/or Pd +Rh on a support material and this in turn being in contact with a substrate.
Additionally, there is a need to reduce the cost of catalytic converter systems. For instance, in automotive exhaust converter systems, since the precious metal (PM) catalysts constitute a significant part of the system cost, it is obviously meaningful to reduce the PM content in catalytic converters. In particular, there is a strong need to reduce the rhodium metal content as this is the most expensive PM presently used. On the other hand, rhodium is an essential constituent to achieve three-way activity (simultaneous conversion of CO, hydrocarbons, and NO.sub.x). The key, therefore, is to develop a low rhodium catalyst system while maintaining three-way activity. This generally means having an acceptably wide redox window for conversion of CO and hydrocarbons to CO.sub.2 and H.sub.2 O, and NO.sub.x to N.sub.2 with little or no ammonia formation. This is important because under reducing conditions, that is at redox ratios of &gt;1, NO.sub.x can convert to ammonia or pass through unconverted if sufficient rhodium is not present or if relatively large amounts of platinum or palladium are present. Since ammonia is itself a noxious gas, conversion of NO.sub.x to ammonia is undesirable.
Also, automotive exhaust emission standards are expected to become increasingly stringent in the future. It is expected that such standards can be met only by cleaning cold start exhaust emissions, that is, emissions occurring during about the first two minutes of engine start-up. Cold start emissions account for about 40-50% of the total emissions in the Federal Test Procedure for passenger vehicles. At present, this exhaust is only partially converted because the catalyst is not up to the temperatures required for effective performance. A need exists, therefore, for a way to clean up cold start emissions as part of catalytic control systems.
It would be an advancement in the art, therefore, to have a three way catalytic system for emission control in which efficient conversion of pollutants is achieved with a reduced rhodium content in the system. Additionally it would be an advancement to efficiently control cold start emissions using a reduced rhodium content in the system.
U.S. Pat. No. 4,071,600 and a related publication entitled "Platinum and Palladium Addition to Supported Rhodium Catalysts for Automotive Emission Control", by James C. Schlatter and Kathleen C. Taylor, Journal of Catalysis, 49, 42-50, (1977) relate to partitioning of rhodium and platinum or palladium catalysts in an automotive emission control catalyst system to minimize formation of ammonia, the rhodium being supported on a refractory substrate.
Japanese patent applications 296,422 and 296,423 relate to catalyst systems for cleaning exhaust gases. A reduction catalyst is used on the exhaust gas flowing-in side and an oxidation catalyst is used on the exhaust gas flowing-out side. The reduction catalyst is stated as being a transition metal on zeolite. The downstream oxidation catalyst is alumina with metals among which can be Pt, Pd Rh, La, and Ce. The system is applicable to leanburn conditions in which production of ammonia is not a problem.